I'd recommend Celebrity Constellation for a 5* experience - she is doing 11n cruise out of FLL next Mar/April. Check out their website celebritycruises.com.
Otherwise Azamara Journey is doing very interesting 12n cruise out of MIA - azamaracruises.com - no children encouraged! Dress code is country-club-casual.

I woulod suggest Ocean Village although it is not a 5* cruise, the service and qualtiy onboard is still very good. It is informal with buffett dining so you don't have to dress up and sit with people you don't know. However if you are celebrating a special occassion you can pay a a small charge to dine in The Bistro. In a 14 night cruise you only have 2 days at sea so you get to see lots of different places. Unfortunetly children are onboard however I hardly saw mine as they were kept occupied in the childrens club.
If having children onboard is a big issue then try P&O's Artemis this ship is child free my parents have travelled on this ship before.